The adapter itself might be the SBS. Or the launcher/BB combo might become a SBS and not a DD as it does not meet the bore size requirement for a DD. Or at least it could be intetpreted that way.

Umm, how does it not meet the bore size requirement? The bore size of a 12 ga is over the 1/2 inch limit that can bring it in to the DD area. The reason why a regular 12 ga shotgun is not a DD is because of the sporting use exemption. I know of no sporting use exemption for the 37mm launchers, so when used with anything beyond a signaling type of ammo, they are considered a DD. Using a 37mm launcher with a 12 ga adapter would still be in the DD area, because as of yet, I have not seen an exemption for this combination listed by the atf under sporting use. Absent of a sporting use exemption by the ATF for this combination, it would be wise to treat it as a DD. At least this is my view, and hopefully you understand the logic behind it.

There are already other 12 ga shotguns that the ATF considers are DD already, and they are factory built shotguns in 12ga, but do not meet the sporting use requirement.

I do see your view point on the SBS aspect, but unfortunatly, I know of no arrest or case law over an adapter alone being a SBS. Perhaps there is case law on this already out there, but I just dont remember any. Others may have more knowledge.